In wireless communication systems, such as defined by IEEE 802.16m specification, base stations and mobile stations communicate with each other by sending and receiving data carried in a series of superframes. Before a mobile station can access a base station, physical (PHY) layer synchronization and Media Access Control (MAC) layer synchronization are performed. Upon power-on, a mobile station first acquires downlink (DL) synchronization and adjusts its timing, frequency and power via synchronization channel (SCH) broadcasted by a serving base station. After DL synchronization, the mobile station acquires uplink (UL) PHY layer synchronization via ranging procedures and MAC layer synchronization via network entry procedures with the serving base station.
A synchronization channel (SCH) is a radio resource region within each superframe that is used for preamble transmission by base stations. A preamble is a predefined code sequence used to facilitate network synchronization. In general, preambles are designed to achieve the following goals: to provide a reliable reference signal in both time domain and frequency domain for network synchronization; to support channel estimation for data and superframe header (SFH) decoding; to provide DL channel quality indicator (CQI) measurement and receive signal strength indicator (RSSI) measurement; to reduce network entry latency; and to minimize communication overhead. Well-designed SCH structure is thus desirable to achieve those goals.
In current IEEE 802.16m wireless systems, a hierarchical two-stage synchronization scheme has been proposed. In a first stage, a primary advanced-preamble (PA-Preamble) is used to provide coarse timing synchronization. In a second stage, multiple secondary advanced-preambles (SA-Preambles) are used to provide fine timing synchronization and cell ID detection. Both PA-Preamble and SA-Preambles are time-domain multiplexed within one superframe. For example, each superframe has four frames, and one PA-Preamble and three SA-Preambles are distributed in each frame within one superframe. It remains a challenge to design a hierarchical SCH structure that reduces network latency and/or provides a desirable tradeoff between network entry latency and the robustness of cell ID detection.